1. Field of the Invention
The present invention relates to a vertical adjustment device for use in a seat in a vehicle such as a truck, and, more particularly, to an improved vertical adjustment device which can be flexed according to the difference between the weights of occupants to adjust its own cushioning property.
2. Description of the Prior Art
A conventional vertical adjustment device of this type comprises a plurality of parallel links provided between a mounting frame to be fixed to a vehicle floor and a seat support frame on which a seat rests, a spring extended between the parallel links and the mounting frame or the seat support frame, and an adjust screw mounted to the spring, whereby the adjust screw can be rotated to adjust the elasticity of the spring. Therefore, when the flexion of the seat is to be adjusted for the occupant's weight ranging from 50 kg to 100 kg, for example, a handle provided integrally with the adjust screw must be rotated 15 times or so for proper adjustment. Also, the frames and links of the conventional adjustment device are complicated in structure, which makes it impossible to reduce the weight of the entire device to a desired level.
Also, there has been disclosed another conventional vertical adjustment device in which a mounting frame is connected with a seat support frame by means of a pair of X-shaped expansion legs and there are provided springs (compression springs) extending between the expansion legs or the seat support frame and the mounting frame. In the last-mentioned conventional device, however, as in the first-mentioned conventional one, a handle must be rotated a large number of times to adjust the elasticity of these springs.